(1) Field of the Invention
The present invention relates to a film forming method for a semiconductor device, and in particular to a method of forming interlayer dielectric films on a semiconductor device which has step units.
(2) Description of the Related Art
A monolithic microwave integrated circuit (MMIC) which is a semiconductor integrated circuit designed for high frequency is formed by integrating active elements such as a field effect transistor (FET) and a hetero-junction bipolar transistor (HBT) into passive elements such as a resistance, an inductor and a capacitor on a single semiconductor substrate. Comparing to a single element, the MMIC can realize a reduction of a price because it can miniaturize and lighten microwave components. Furthermore, there is no need to connect between elements with a golden wire in the MMIC so that the high frequency characteristic is improved.
A thin film resistance by metal such as tungsten silicon nitride (WSiN) is used for the resistance that is one of the components of the MMIC. The thin film resistance is formed by forming metal into a film using a sputtering method and patterning the formed metal by dry etching using chlorine gas. At this time, the thin film resistance is formed on a dielectric film such as a silicon oxide film (SiO2) to separate the thin film resistance electrically from other elements. However, in the case where elements such as FET and HBT are previously formed, the metal remains around a step unit of the element and cannot be removed by dry etching.
FIG. 6 is a cross-section diagram of a semiconductor device for explaining a problem of metal remaining in the case where this conventional thin film resistance is formed.
A metal remaining 604 which cannot be removed by dry etching occurs around a step unit 605 on a substrate 601 by depositing a metal thin film on a dielectric film 603 and forming a thin film resistance 602 by the dry etching when there are surface steps such as a step unit 605 on the substrate 601.
To avoid the problem, for example, there is a dielectric film forming method using SiO2 film (hereafter referred to as ozone TEOS (Tetraethylorthosilicate) film) that is formed by a normal pressure CVD method using mixed gas of organosilane with ozone (refer to Japanese Laid-Open Patent Publication application No. 3-123029). Compared to a SiO2 film formed by a conventional plasma CVD method, the ozone TEOS film has good step coverage, and a surface step can be moderated. Therefore, the problem of metal remaining around the step unit can be avoided by using an ozone TEOS film as a base dielectric film of the thin film resistance.
However, the conventional dielectric film formation method has a following problem. The ozone TEOS film changes its step coverage according to ozone concentration and the step coverage is improved as the ozone concentration is higher. However, if the ozone TEOS film is formed using ozone with high concentration to moderate the step, the surface of the ozone TEOS film becomes rougher. As the result, the resistance value of the thin film resistance on the ozone TEOS film changes so that a designed resistance value cannot be obtained.